1. I'm convinced believer's baptism is a more biblical practice than infant baptism.
As I read the NT, the silence on the issue is one of the first things that strikes me. But as you read about baptism in the NT, its presentation as a sign of repentance and faith which follows repentance and faith in response to the gospel is clear. I remember what pushed me over the line on this issue, it was reading Piper's "Brothers, We are not professionals". The difference between Old Covenant and New Covenant membership is definitive for me on this issue.
More pragmatically though, I'm a bit of a wheneverist on the issue. I don't believe in rebaptism of those baptised as infants, because baptism is once for all, and if someone was baptised as a child, is living a life of faith and repentance, we don't need to get them baptised 'properly', as if the sacrament was ineffectual. I think we need to be more biblical in encouraging believers to get baptised, and less caught up in debates that often involve an ontology we're not otherwise committed to.
2. The 3-fold order is biblically unconvincing.
At least from the NT. The terms for bishop and presbyter clearly overlap in usage, and deacons are not what they are now. I'm not opposed to the 3-fold order, especially given its historical emergence in the early church, which I have no problem with, I just can't insist on it as a distinctive.
3. Normative, not Regulative, principle.
The Presbyterian/reformed tradition is totally unconvincing to me on this issue. I think this is shifting in a lot of those circles though. A good Missiology should disabuse us of the Regulative principle straight up, and if not, we need to think a lot harder about culture & gospel.
4. Pacifism & Community
Are the two key things the Anabaptists got really right (well, not all the historic Anabaptists got Pacifism right). I recognise the idiosyncrasy and minority theological position of pacifism within contemporary Christianity, but I find the ethical conclusion from reading the scriptures inescapable on that issue. (Yoder was right!)
As for community, the corporate emphasis of life in Christ is often underplayed and under-resourced in other traditions, which plays to individualism and self-worship. Anabaptists on this score get high marks.